Standard Units

This page is intended to be a brief summary of our present knowledge about
the units of measurement employed in the STAR WARS galaxy.
Some of the following information is derived directly from the films;
this is irrefutable canon.

Some of it depends on planetological and other data printed in
references from STAR WARS: The Roleplaying Game.
The original invention of those figures was probably arbitrary,
and the consequent results presented here
are only as solid as the reader's willingness to accept the RPG material.

Thanks are due to, in alphabetic order:

Dave Baughman
for suggesting MGLT as a measure of uncompensated acceleration.

Robert Brown
for the details of the SW Monopoly coins,
for various general discussions,
for proposing the calaculation of the realistic MGLT,
and for the suggestion of the image-link icon.

Stewart Brown
for information about the use of "megalight"
in X-Wing: Alliance.

Scott Griffith
for requesting refinement of the explanation of weight.

The units of distance measurement
employed in the STAR WARS galaxy are metric,
based on the metre.
In the A New Hope novel, for instance,
Darth Vader is described as being over two metres tall,
lightsabre beams are a metre long,
and womp-rats are two or three metres in size.

Starfighter pilots and their targeting computers use metric metres.
[Eg. Luke Skywalker's reference to womp rats in A New Hope;
General Dodonna's reference to a thermal exhaust port.]
The number of digits on the targeting displays for rebel X-Wing
proton torpedos indicate that these weapons have an effective range of
up to a million metres; a thousand kilometres.
The reason why pilots at the Battle of Yavin were unable to fire until
almost zero-range is because they flew near the station's surface.
Flying near the surface put the target exhaust port over the horizon.
Approaching the port from above the station's surface would have denied
the pilots cover from the surrounding turbolaser batteries.
With anything other than the trench-strafing approach,
they would have been shot down before gaining a target lock.

Different units are used in flight simulation computer games like
X-Wing and TIE Fighter.
Although the dimensions of starships in the games are stated in terms of
metres or kilometres,
an unstated new unit is used to measure separations between vessels.
However, as noted in the propulsion section below,
the dynamics of these games are not quite the same as in the "real"
STAR WARS universe,
and the units have been contrived to maintain playability.

For special purposes, such as interstellar travel,
there are supplementary units in use.
One is the parsec,
which is an astronomical unit characterising the distances between stars.
A parsec is defined to be the distance at which
the average separation between Earth and Sun
would subtend an angle of one arcsecond (1/3600 of a degree) on the sky.
The corresponding unit in the Galactic Empire would be based on the
orbit of Coruscant rather than Earth.
We don't yet have any way of determining what that measurement is;
it could just as easily be slightly less or slightly greater,
depending on the mass of Coruscant's sun.
We provisionally assume that the orbital semimajor axis is the same.
A terrestrial parsec equals 3.086x1016m.
Han Solo boasted that his freighter, the Millennium Falcon,
was capable of accomplishing a feat called
"the Kessel Run"
in "less than twelve parsecs".
(The 'Run involves travel through or past an unnavigable region;
taking a route of shorter distance requires a good ship and skillful pilot.)

A light-year is the distance travelled by light in one year.
An Earth-based light-year is 9.461x1015m.
However this is not appropriate in a STAR WARS context,
because the galactic standard year is derived from the orbital period
of Coruscant rather than Earth.
As stated below in the time section,
the standard year is slightly longer than the terrestrial equivalent.
We assume that the standard second exactly equals the usual metric second.
Therefore when a citizen of the Old Republic, Galactic Empire or New Republic
quotes a distance in "light-years",
the unit is actually 368/365.25 times our version of the light-year.
(It is about 0.75% greater.)
By the Coruscant definition, a light-year is approximately
9.531x1015m.
In Shield of Lies,
Lando Calrissian states that the galaxy is 120,000ly in diameter.

X-Wing proton torpedo target scope has six digits;
according to the scale of the battle station and the length of the trench
(roughly one eighth of the global circumference)
the units of measurement are almost certainly metres.
Furthermore the pilot briefing given by General Jan Dodonna was
expressed in terms of metres.

The galactic standard second appears to be identical to the metric second.
Higher units of time are reckoned according to the clock and calendar of
Coruscant.
A standard minute consists of 60 seconds;
an hour consists of 60 minutes;
a day consists of 24 hours.
A standard week consists of 5 days;
a month consists of seven weeks.
A standard year is 368 days,
including ten months, three fete weeks and three other holidays.

The dating systems of the Galactic Republic and Galactic Empire are unknown.
At least two proposals for dating standards have been used in non-canonical
literature.
Both are unsatisfactory:

In Timothy Zahn's novel Dark Force Rising, a calendar based on the
Emperor's rise to power was used. Unfortunately there are a number of
continuity problems with Zahn's dating.
The worst problem is that he has
Vader's adult career well underway several years before Episode I,
in which he will be a ten-year-old boy.

In West End Games' Star Wars Adventure Journal
there is a regular series
of newspaper-style articles using a dating scheme which puts
The Empire Strikes Back
at around year 37.
However this cannot correspond to
the Emperor's rise to power,
since Episode I was only 35 years prior to
The Empire Strikes Back,
and the takeover probably takes place in Episode III
(only 23 years prior to TESB).

Nothing is known with certainty about the units of mass
used in Palpatine's galaxy.
By default, we should assume that the base unit is the metric
kilogram.

At first thought, some might wonder whether the kilogram would be redefined
because of Coruscant's surface gravity being higher than Earth's.
This is just a confusion between the concepts of mass and weight.
Weight is the force on an object due to gravity;
mass is the amount of substance in the object.
Force is mass multiplied by acceleration
(such as the local acceleration due to gravity).
Weight is equal to the mass times the strength of the local gravitational
field.
The metric unit of force is the newton (N),
which equals 1kg.m/s².
Obviously a different name would be used for the same unit
in STAR WARS,
since Sir Isaac Newton probably isn't remembered in that galaxy and era.

Mechanical scales do not directly measure mass.
They measure the weight force.
The mass reading is given according to a calibrated scale
which assumes that the local gravity has a certain fixed value.
The definition of mass itself is quite independent of weight
and a planet's gravitational conditions.

Money in the Old Republic, Galactic Empire and New Republic
takes the form of
"galactic credits standard".
This is a universal unit of currency.
(For a few years after Palpatine's fall, though,
there were competing New Republic and Imperial currencies.)
In A New Hope, credits took the form of
coins (as thrown from Han Solo to a bartender).
The STAR WARS Monopoly CD-ROM states that although
the galactic credit is standardised,
local mints (presumably operating at a planetary or sector level)
produce coins bearing regional symbols and labels.
Roleplaying sources state that sophisticated electronic devices
called credit chips are also used to record credit transactions.
Credit chips are supposed to have nearly impregnable security and encryption.
Credit notes akin to those used on Earth are not known.

Han Solo took his reward from the Yavin rebels in a form contained
in heavy boxes of approximately the same dimensions as a shoe box.
The total capacity of all these containers looked to be at least a ton.
The material within the boxes was probably a commodity with a stable price;
ordinary credit coinage or chips surely would not be so voluminous.
The radio dramatisation suggests that the material was a precious metal.
Solo and the rebels probably chose this form of transaction because
actual credits could potentially be traced by Imperial Intelligence
analysts.

There are very few canonical indicators of the value of the galactic credit.
Ten thousand credits is considered to be excessively expensive
as a fare from Tatooine (in the Outer Rim) to Alderaan (a Core world).
A modestly priced transgalactic voyage along a fast hyperlane
would take a few days and cost only a few thousand credits.
A second-hand hyperspace-capable starship costs more
(but not substantially more) than ten thousand credits.
Luke Skywalker sold his worn-out landspeeder for approximately 2000 credits
(the advance fee demanded by Han Solo for passage to Alderaan).
Jabba the Hutt was known to pay 35000 credits bounty
for live delivery of one of his wookiee debtors.
Drinks at a Mos Eisley cantina cost several coins' worth,
but the credit value of the individual coins is unknown.

There are numerous statements of credit value in official
(but non-canon) sources.
The roleplaying game has extensive tables of pricing.
In novels, the training of a stormtrooper officer costs over
half a million credits;
ten million credits buys a wing of starfighters;
C-3PO objectively values himself at 43000 credits,
and his constituent parts as five thousand.
A "decicred" coin, which is valued at a tenth of a credit,
is flipped for gambling purposes
[Rebel Dawn p.310].

There is zero nett inflation in the galactic economy.
The credit retained roughly equal value over at least four millenia.
Compare stated prices of goods and services stated in
Tales of the Jedi Companion
with prices for similar things in
the films, unfilmed fiction and roleplaying material of the Palpatine Era.
The absence of inflation for over 160 generations
indicates extraordinary stability which deserves detailed examination.
This fact is a key hint at the nature of the galactic economy.
This should be discussed elsewhere.

Credits paid by Han Solo to the cantina bartender
appeared to be rectangular metallic coins of some kind.
A similar transaction took place when Luke Skywalker sold his landspeeder,
but unfortunately Luke and the buyer were too far from camera for the
exchanged tokens to be discerned.
However, the exchange did involve items fitting into the palm of a hand,
which were probably coins.

Images of various 1000-credit coins from the STAR WARS Monopoly
game.
The two lowermost golden coins bear the starburst in diamond logo
of the Corporate Sector Authority.

Wupiupi coins of Tatooine from The Phantom Menace,
during the period when Hutt syndicates dominated the planet
and controlled its economy by issuing their own hard currency
in opposition to the credits of the Old Republic.

Han Solo's reward from the rebels at yavin took the form of a bulky commodity
stored in boxes.
In the A New Hope radio dramatisation
it was suggested that the goods were rare metals.

Bounty hunting is an important profession in the lives of disreputable
citizens of the galaxy.
Gangsters are willing to place generous bounties on their debtors.
Even senior advisors of the Emperor are not above hiring mercenaries to
capture important subversives and criminals.
The hunters are able to pursue their prey across the galaxy
and make a living for rewards of only tens of thousands of credits
(e.g. Chewbacca's reward in ROTJ).

Both the flight-simulation computer games
and the starship rules of the roleplaying game
sacrifice realism in propulsion for the sake of
the commonsense intuitions of air-dwelling players.
The games implicitly assume the dynamics of vehicles
flying in a resistive medium.
In other words, it appears as if maintaining a velocity requires effort.
In real empty space an object remains in motion at constant speed and direction
until it is acted upon by a force;
it does not dwindle to a halt when thrust is turned off.

A starfighter's drive systems actually cause
straight-line acceleration, deceleration,
and/or changes of direction and orientation.
For realistic technical purposes
the games' speed statistics should be regarded as either acceleration,
thrust or engine power;
the "acceleration" statistics
are actually the ability to vary acceleration;
and the manoeuvrability statistics should remain as measures
some kind of maximum manoeuvring torque.

The propulsion statistics are usually stated in terms of arbitrary
game-specific units that cannot be readily related to the real world.
The computer games use a "speed" unit called the MGLT,
which looks like it might be an acronym.
The roleplaying game does not even label its distance and speed units.
In fact the roleplaying units are said to take different values depending on
the difficulty of movement due to local conditions, hazards and obstructions.
The statistics are simply meant to convey the idea that some vessels
are more nimble or are capable of outrunning other kinds of vessels.
The only exception, when speeds are stated in terms of absolute
metres per five seconds
or kilometres per hour,
is for atmospheric or ground movement.

The MGLT was used (and probably invented by) ILM artists long before
the development of the modern computer games.
The earliest known use of the term is in a diagram comparing the
propulsive capabilities of starfighters,
drawn during the production of Return of the Jedi.
This illustration is reproduced in Return of the Jedi Sketchbook.
As an acronym, the unit probably refers to something in the production
of the movies' special effects;
the "GL" could stand for "George Lucas".
As a measurement of the maximum straight-line acceleration
of a starship in space, it might (speculatively) be an acronym for
something like "Micro-Gravity Linear [acceleration] Tolerance",
as suggested by one reader.
The instruction booklet for the game X-Wing: Alliance
uses the phrase "megalights per hour"
which is probably a colloquial second-generation attempt to rationalise
the MGLT by inserting vowel sounds,
but this might not have been the original ILM intention.

Judged on this basis,
rather than the unrealistic kinematics of the computer games,
the MGLT must be a measurement of acceleration.
Because a star destroyer can keep pace with the Millennium Falcon
in a straight-line chase,
we know that it must have approximately the same statistic:
75 MGLT,
or better if Solo was stressing his freighter beyond its safe limits.
Since the Imperial fleet in Return of the Jedi
demonstrated a deceleration of not less than 30km/s²,
[see Star Destroyers]
1 MGLT must equal at least 400m/s².
This is about 40g in terms of Earth gravity,
or 29gs in terms of Coruscant gravity.

A similar theory is that the MGLT may describe the residual acceleration
felt by the crew of a spaceship,
reduced from the full inertial force by the action of
acceleration compensators (inertial dampers).
This would complicate the specifications by convolving
the propulsive capabilities with the precision and capacity of inertial dampers.
The value of 1MGLT would have to be significantly less than
the acceleration due to a standard gravity,
because starfighters are known to perform at over 100MGLT
and yet acclerations greater than a few standard gravities
are harmful to human health.

Comparing the MGLT propulsion ratings from LucasArts and ILM
with the roleplaying game statistics is not trivial.
By performing regression analysis on the data for several known
rebel and Imperial fighters and bombers,
it is possible to derive approximate mathematical expressions relating
these three different acceleration ratings.
The ILM rating in MGLT is proportional to
the LucasArts rating raised to the power of 2.4.
Thus the TIE Avenger and Defender are 240MGLT and 282MGLT on the ILM scale.
The WEG rating is weakly related to the other standards.
If the LucasArts MGLT rating is L
then the WEG rating is in the range W±1,
where W=0.14L-5.5 approximately.
Thus the WEG propulsion ratings of the Avenger and Defender
cannot be less than 14 and 16 respectively.

It must be stressed again that the use of speed-based statistics
rather than realistic accelerative propulsion
is simply a convention for the comfort of players.
Other statistics in the computer games,
including hull, shield and manoeuvrability codes,
are also meant to imply that starships have different capabilities
but may reflect the realistic STAR WARS universe only as well
as the propulsion figures.
Nevertheless, it is useful to consider what the game characteristics
are,
as well as what they realistically ought to be.

The magnitude of the MGLT can be determined by
flying past large warships of known length
at a fixed speed of 100MGLT
whilst making careful timing measurements.
Charbel Tengroth has performed these experiments and found that
100MGLT = 100m/s,
and the relationship appears to be linear in the interval of 50-150MGLT.
So in terms of gameplay, 1MGLT is one metre per second.

Makis Kalofolias has tested the game program further to determine that the
game-speed MGLT equals 1m/s under all circumstances.
He also finds that the internal accelerations statistics,
expressed in MGLT/s
are the same as the metric acceleration unit 1m/s².

His investigations also show that
manoeuverability is actually two characteristics:
a pitch rate (P) and a roll rate (R).
The overall manoeuverability statistic is R+2P,
and is measured in a unit called DPF.
The meaning of this acronym is unknown,
but it might be "degrees per frame" or something similar.

Mr Kalofolias also calibrated the units for
armoured durability of hulls
(RU, possibly "resistance units"),
shield base durability (SBD),
and the potency of weapons.
It turns out that 1RU has the same effectiveness as 1SBD
for resisting damage by starfighter weapons.
That is the ideal behavior, but the game engine introduces some irregularities
in order to provide a subtle sense of variation in the simulated combat.
The unnamed units of weapon damage,
in terms of which an ordinary proton torpedo is 10000,
are destructively equivalent to 0.01 of a RU or SBD,
ie. 1RU = 1SBD = 100 units of weapon destructiveness.
If we define a basic unit of destructive potential (DP) equal to 1RU or 1SBD,
some useful statistics for comparison are:

In TIE Fighter the player is often allowed the choice
of missile payload for missions invoving bombers or deluxe fightercraft.
In this interface, blast ratings and propulsive capabilities are listed
for each option.
These statistics are roughly equivalent to those presented above
based on internal inspection of the game engine.

Standard values refer to the planet Coruscant,
which is the capital of galactic civilisation
and is considered the ideal inhabitable world for humans.
Galaxy Guide 2: Yavin & Bespin
claims that standard planetary density is 5.63 times that of water;
ie. 5630kg.m-3.
The book also contains several definite statements of measured statistics
of particular planets and moons.
Combining these figures with the known universal physical constants
allows us to deduce the real values of the rest of the
galactic standard units for planetary measurement.

The gas-giant planet Yavin has
radius R=96239km,
mass M=323.7Ms,
and surface gravity g=2.74gs.
Its year is slightly greater than 13 standard years,
and its sidereal day is slightly less than 12 hours.
The system is 7.5 billion years old.
Combining the statistics for surface gravity and planetary mass
yields an equation relating the standard units
of surface gravity and planetary mass.

gs / (m.s-2) = Ms / (1.175x1024kg)

Taking note of the fact that
standard surface gravity, planetary mass
and planetary radius
are actually the exact properties of a single planet, namely Coruscant,
the standard values must equal:

gs = 13.93 m.s-2
Ms = 1.637x1025kg
Rs = 8854km

For comparison, the mass of Earth is
5.974x1024kg,
and its average surface gravity is 9.807 m.s-2.
Therefore, Coruscant's gravity is 1.421 times that of Earth;
its mass is 2.74 times Earth's;
and its radius (and diameter) are 1.389 times Earth values.
The surface area of Coruscant is almost double the surface area of Earth
(1.929 times).

On a miscellaneous note:
antigrav range for a habitable planet is within
approximately six planetary diameters.
In other words, within this range repulsorlift is the propulsion method
of preference.
For an Earth-sized body, this range is just under eighty-thousand kilometres.

For further information about the filmed planets of STAR WARS,
refer to Planets.

The gas-giant planet Yavin; a vital yardstick for determining the
standards of planetary characteristics in STAR WARS.

Coruscant (also known as Imperial Centre) as seen from space glitters and shines like a brilliant jewel,
due to the refraction and reflection of sunshine off
the countless angular surfaces of the city-covered globe.
This planet is the standard by which other worlds are measured.

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